| 摘要 |
PROBLEM TO BE SOLVED: To surely decide the state of super-imposed character data during the prescribed horizontal scanning period of video signals by shifting a 2nd reference voltage just for +αV according to a resistance ratio. SOLUTION: A 1st comparing means 4 compares the level of character data superimposed during the prescribed horizontal scanning period of video signals with a 1st reference voltage. Based on this output, MOS transistors 6 and 7, resistor 9, capacitor 10 and resistor 11 are charged/discharged and the 1st reference voltage is varied. Resistors 14 and 15 generate a 2nd reference voltage VREF2 lower than the 1st reference voltage. Resistors 18 and 19 connected between the output of a 1/2 VDD voltage generation circuit, which is composed of resistors 1 and 2 and an amplifier 3, and a VDD are operated for shifting the 2nd reference voltage VREF2 so as to increase it. Thus, since the VREF2 is set lower than the maximum level of caption data and higher than a noise level at the time of superimposing no caption data, malfunction can be surely prevented. |
